Seismic detector



IVENTOR.

Roberi L. Henson w fig ATT0R 5 5 SEISMIC DETECTOR Filed Feb. 4, 1.947

R. L. HENS ON JR //////////A////// 1 I] I l M Ill. |.l| l l .l ?1 IF 3 mM Dec. 12, 1950 Patented Dec. 12, 1950 PAT E'NT OFFICE SEISMIC DETECTORRobert L. Henson, Jr., Beaumont, Tex, assignor to Sun Oil Company,Philadelphia, Pa., a corporation of New Jersey Application February 4,1947, Serial No. 726,252

This invention relates to a seismic detector and has particularreference to an improved detector of astatic type which will avoidpickup of stray magnetic fields.

Detectors such as used in seismic prospecting work are required to be ofhigh sensitivity in view of the fact that they must give useableelectrical outputs for extremely minute movements of the earth resultingfrom seismic waves emanating from a distant explosion. Such high degreeof sensitivity is ordinarily accompanied by an equally high tendencytoward the pick-up of stray magnetic fields due to power lines or thelike giving rise to signals usually in the nature of hum of frequenciesin the range of the earth frequencies to which the detector is normallysupposed to respond.

In accordance with the present invention there is provided an improvedseismic detector of simple construction and of high sensitivity coupledwith astatic properties which reduce to a very low level its pick-up ofstray magnetic fields.

The accomplishment of this result as well as other objects of theinvention particularly relating to details of construction will becomeapparent from the following description read in conjunction with theaccompanying drawing in which:

Figure 1 is a vertical axial section through the improved detector;

Figure 2 is a transverse section taken on the plane indicated at 2-2 inFigure 1; and

Figure 3 is a plane elevation of one of the suspension springs for themoving system.

Within a cylindrical housing 2 of non-magnetic material there is locateda powerful permanent magnet in the form of a cylinder 4 which may be ofAlnico or the like. Associated with the ends of this are disc-shapedsteel pole pieces 6. A central steel core 8 has fitted on its end steelrings l and the assembly thus provided is secured in concentricrelationship with the parts first described by means of spacers l2 ofnon-magnetic material, for example, aluminum, which have press fits onthe core 8, and fit into shallow recesses in the pole pieces 6. Theouter pole pieces 6 may be welded to the magnet 4 while the spacer ringsare deflected slightly by pressure applied to the pole pieces 6. Thearrangement,

9 Claims. (Cl. 171209) as will be evident from Figure 1, provides a pairof annularair gaps between the respective members 6 and Ill. The magnet4 having its poles at its ends thus provides a permanent magnetic fieldextending in an inward radial direction across one of these gaps and inan outward radial direction across the other.

A suspended system includes the rod l4 of non-magnetic material to theends of which are clamped by nuts IS a pair of coil forms l8 havingflanges extending through the air gaps and arranged to support thepick-up coils 22. All of these various parts are of non-magneticmaterial. In particular, the coil forms are formed of anodized, andtherefore surface insulated, aluminum alloy. At the ends this assemblyis resiliently mounted by means of a fiat spring arrangement indicatedat 24 in Figure 3. As illustrated in that figure an annular ring 26 isprovided with fingers 30 which support a central portion 28 clamped tothe moving assembly by nut I 6. The clamping is elfected betweeninsulated washers and 21. Upper and lower connecting lugs 29 and 3| areclamped between the coil forms and washers 21. The member 24 ispreferably formed of a spring material such as beryllium copper in theform of a thin stamping to supply the necessary resiliency.

As will be evident, the entire structure is symmetrical about thecentral axis and consequently the introduction of any stray magneticfield causes substantially symmetrical changes in the whole system.

The coils 22 are so wound and connected that for a given relativemovement of the parts of the device the electromotive forces induced inthe coils are added, each of the coils, which are identical in number ofturns and dimensions, thus contributing one half the usefulelectromotive force output. As will be evident, however, since the fieldis inwardly directed across one gap and outwardly across the other, thecoils buck each other to the extent that they may be threaded by anexternally arising stray magnetic field, i. e., assuming a verticalcomponent of such a field threading upwardly through the apparatus,variations therein will produce bucking electromotlve forces in thecoils which balance out. The result accordingly is a detector whichstray field while at the same time it has the. rev

quired sensitivity to earth movements which occasion'movements of theframe of the detector 1 while the coils remain stationary.

The actual physical electrical connections through the coils aremadefrom the terminal 38 through a. flexible pigtail 40 to the lug 29 whichis electrically connected to one end of ship, the central "coreelementsaid annular pole pieces providing a pairgot symmetricallyarranged annular air gaps, aniarmature including a'pair of coils locatedwithin said gaps, and spring means supporting said armature, thearmature extending through said central core element, said coils beingastatically arranged so that stray the upper coil M. The otherend ofthis coil is connected through rod l4 to-an end of lower.

coil 22 and the other end of the latter is conthe magnet assembly areadditiver' A nected by a flexible pigtail and a lead (notf' shown)threaded upward through the magnet assembly tothe terminal 42. Externalconnections are madefrom the terminals 38 and 42.;

In addit on to the foregoing, it will be n'otedv that effective dampingis provided by forming the coil forms of aluminum orother non-mag neticmetal so that their movements inthe stron magnetic field will produceelectromagnetic damping. Thus there may beavoided damping liquids withtheir attendant disadvantages.

magnetic'fields induce therein bucking electromotive forces, whileelectromotive forces induced therein by reative movements of the coils"and 5. Aseismic detector comprising a-cylimtirical I permanent magnet,annular pole I pieces; associated therewith, a central, core'element-within said permanent magnet, 'iion magnetic spacer ringsengaging said core elementand annular pole pieces to maintain them in"concentric relationship, the central-core "element and said annuiar polepieces providing-a pair. of symmetrically arranged annular air gaps, anarmature in- 'cluding a pair of coils; locatediwithin said gaps, andflat spring means supporting said armature,

It will be clear that numerous variat ons may be made in details of theap aratus without de-x parting from the. scope of the invention asdennedin the folowingclaims.

What I Patent is:

1. A seismic'detector comprisingmeans providing a rigid permanent magnetassembly. having a pair of symmetrically arranged'annular a rgaps. anon-magnet c armature including a pair of coils and conductive elementsmounting sa d coils lo ated within said ga s. and scring meanssupporting sa darmature, said coils bein astatlclaim and. desire toprotect by. Letters cally arranged so that stray magnetic fie ds cut-.

ting both cois induce therein bucking e ectromotive forces. whileelectromotive forces inducedsaid coils being astatic'ally arrangedjs'othat stray magnetic fields induce, therein bucking; electromotiveforces, whileelectromotive forces induced therein by relative movementsoffthej coils and the magnet assemb y are additive 6. A seismic.detectorw'comprisingia cylindrical permanent mag1'iet,-.ann u larvpole*piecesj. associated therewith,a central core element within saidpermanent magnet; nonmagnetic-spacer rings engaging's'aid coreeler'ne'nt-andannular polepieces to maintain them' in concentricrelation ship, the central core element and said. annular pole piecesproviding a pairp! symmetrically arranged annular air gaps, an armatureincluding a pairofcoils' located within said gaps. and

therein by relative movements of the 'coils and the n' agnet assemblyare add tive. I

viding a r g d permanent magnet assembl hav- 40' 2. ,A seismic detectorcomprising: means, pro? ing a pair of symmetrically arrang d annular airgaps, a non-magnetic armature including a pa r of coils and conductiveelements mounting said coils located within said gaps, and flat s ringmeans support ng said armature, said coi's being astatically arran ed sothat stray ma n tic fields cutting both coils induce therein bucking elctromotive forces, wh le electromotive for es induced therein byrelative movements of the coils and the magnet assembl are additive.

spring, means supporting saidf-armature; said coils. being astaticallyarranged so that stray motive forces,'while electromotiveforces inducedtherein by relative movements of thecdfls and the magnet assemby areadditive.

7. A seismic; detector comprisinga. d'rical permanent magnet and polepieces.- associated therewith toprovide a pair'ot annular air gapssymmetrically, arranged about a; common axis, anon-magnetic armatureincluding a pair 3. A seismic detector comprising a cylindrical Ielectromotive forces, while e ectromotive forces of coils andconductive. elements mounting said .coils located-'within, saidfiairgaps, and spring means supportingsaid armatures, said coilsbeingastatically arranged so-that. stray magnetic fields cutting both coilsinduce therein bucking electromotive forces, while electromotive forcesinduced therein by relative movements of the coils and the magnetassembly are additive,

a 8. A seismic detector comprising a rigid cylindrical permanent magnetand pole' pieces 'assocl induced therein by relative movements of the: I

cois and the magnet assemblv areaddit ve. I

4. A seismic detectorcomprising a cylindrical," permanent magnet,annular p'oe 'pieces associ ated therewith. a central core elementwithin said permanent magnet, non-magnetic spacer rin s engaging sa dcore element and annular pole I pieces to maintain them in concentricrelationated therewith to provide'a pair'ofannular air gaps symmetricalyarranged 1 about a .common axis, a non-magnetic armature includinga'pair of coils and conductiveflelements' mounting said coils located:withinsaid gaps, and flat spring means supporting said armaturefs'aidcoils being astatically arrang'ed sothat straymagnetic fields cuttingboth coilsinduce' therein hucking electro-.

- motive.. orces, while electr'orhotive forces, induced therein'byreative movem nts of the coils and the m'agil'e assembly areadditive.

- 9.. Agseis'm'ic. detector comprising"inea'ris pro yidin'gj a'zrlg dpermanent:rna gnet'assemblv having a pair ofannularf'airgapssymmetri'callvarranged 1 about ai common axis, a "nonemagneticarmature inc uding a'nair of coils and conductive elementsmounting saidcbilsylocated within said magnetic fields induce. therein :biicking'elect'ro rigid cylin-' 5' a air gaps and flat spring mounting meanscomprising a perforated metal disc having a center portion, supportingsaid armature, connected to an outer portion, mounted to said permanentmagnet assembly, by arms of lengths greater than 5 the radius of thedisc, said coils being astatically arranged so that stray magneticfields cutting both coils induce therein bucking electromotive forces,while electromotive iorces induced therein by relative movements of thecoils and the m magnet assembly are additive.

ROBERT L. HENSON, JR.

6 REFERENCES crrnn The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,906,066 Holst et a1 Apr. 25,1933 2,254,172 Draper Aug. 26, 19% 2,296,754 Woliet a1. Sept. 22, 1942

